The use of wall dimmers, to provide controlled light output from a luminaire, have been in use for quite some time. Many varieties of dimming for mains AC are available, however the most popular to date are known as leading edge and trailing edge dimmers. This technology was developed in the early 20th century and relies heavily on the load, in most cases an incandescent bulb, to provide a sufficient load to correctly trigger and latch the dimmer's operation. The leading and falling edge dimmers are typically a silicon controlled rectifier (SCR) solid state device that may be latched by break-over voltage or by exceeding the critical rate of voltage rise between anode and cathode, just as with the Schottky diode. Leading edge dimmers will chop off the leading edge of the sine wave and trailing edge will chop off the trailing edge of the sine wave.
The current through the dimmer circuit controlling the SCR also controls the trigger mechanism via a RC network, where the resistance is the actual load (the globe) itself. This means that if the impedance is too high or the load is capacitive or inductive, the RC network/trigger level is unable to phase-shift the threshold significantly, and in some cases even becomes unstable, resulting in flickering (hence the reason why most existing energy saver globes do not dim effectively with existing infrastructure.)
The proliferation of low voltage (12v) 20 w-50 w dichroic halogen down lighting in world markets has driven the cost of manufacturing both the dimmer and the transformer (takes 240 VAC mains and converts it to 12 VAC) down to a point where they provide very ‘dirty’ power to the down light. The installation of such systems in commercial, industrial and residential premises requires careful selection of the transformer and dimmer as incorrect configurations can cause damage to both the dimmer, transformer and down light.
Energy efficient lighting places a much smaller load on both the dimmer and transformers (typically <10 W) resulting in the dimmer and transformer operating outside of specification. This results in instabilities which include;                1. Low load current causes the dimmer to operate below the specified minimum, causing poor dimming range and potential oscillations in the triggering of edges, which is visible in the globe (flickering.)        2. The instabilities on the leading and trailing edge can cause catastrophic inductive power spikes with magnetic transformers which can damage both the transformer and light        3. Many electronic transformers require low impedance loads (20 W-50 W) on its output to dampen the switch mode output of these devices and maintain stability        4. Providing a low impedance load is critical for many electronic transformers as they will “sense” the output current load to ensure its within a specification they have been designed for (typically, but not limited to 20 W-50 W)        
There are three currently independent technology fields of concern: Cold Cathode Florescent Lamps (CCFLs), Light Emitting Diodes (LEDs) and halogen down light systems.
Cold Cathode Florescent Lamps (CCFLs) produce either a specific wavelength of light (such as red, green, blue, UV etc) or a certain bandwidth (warm white, hot white, blue white) without the need for the traditional heating element or filament found in normal florescent and incandescent lights.
Perhaps a predecessor, Compact fluorescent lamps (CFLs) are simply traditional fluorescent lamps folded or twisted into a compact form. The technology is more efficient than incandescent globes—primarily because CFLs do not require a filament to be heated to over 3000 degrees Kelvin. The existence of a hot filament is the primary cause of excessive heat, and over time the filament will fail either due to evaporation, or by mechanical stress caused by repeated heating and cooling as the light is switched on and off.
Unlike CFLs, Cold Cathode Florescent Lamps (CCFLs) have no filament, instead relying on excitation of the phosphor coating. CCFLs can provide over 100 lumens per watt depending on configuration, and last 20,000 hours or more due to the lack of heating filament fatigue. An AC voltage source of sufficient magnitude and frequency is necessary to excite the ions sufficiently enough to produce the desired light. Current in CCFLs is typically small—usually below about 6 mA. Optimal efficiency is achieved when the source frequency is above 10 KHz.
CCFLs have been used commercially for nearly 20 years, and can be found most commonly in LCD screens such as flat screen televisions and laptops. As the light output in these globes is so efficient, care must be taken to ensure that constant light is achieved by ensuring a stable power supply. Whilst light is emitted within microseconds of power being applied, the luminaire itself will warm up, getting brighter as the negative impedance behaviour after striking results in more current for a given voltage.
Commercial applications such as LCD televisions normally have complex control systems incorporating light and/or current sensors which ensure that light output is stable and controllable.
Typically a CCFL Tube has a diameter of 2 to 5 mm and tube length of 100 mm to about 500 mm. It typically requires an inverter to increase input voltages usually between 5 and 25 V with an output voltage of inverter of 400v to 1200V and globe current draw of about 5.0 to 6.0 mA. This produces a brightness of 18,000˜30,000 cd/m2 with a lifetime of some 30,000 hours, depending on manufacturer. It therefore provides high brightness, long lifetime, high reliability and easy installation.
LEDs are non-linear silicon-based PN junctions designed to emit a certain frequency of light when electrons jump a specific energy band gap when voltage is applied. The result is a narrow wavelength of light, completely selectable from IR to UV. However, this does cause a problem if broader spectrum light such as white or warm white is desired. Re-transmitters are required, such as phosphor coatings but these have only limited success. Another issue is that producing large amounts of light as necessary in Halogen light markets require relatively massive emitters, which are formidably expensive, and require extensive heat dissipation to prevent destruction of the device by the heat stress caused by ohmic losses at large currents.
Another field of low power lighting are halogen downlights which are traditionally 12 volts as the original filament technology was not easily achieved with mains power (110 to 220V.) Using a complex system involving heat reflectors, UV filters, and filament maintaining halogen gas, halogen globes achieve slightly higher lumens per watt and life expectancy than traditional incandescent globes. Part of the incandescent family, the lamps use super-heated filaments which emit light according to the filament's physical temperature. Whilst operation is very simple, the bandwidth of electromagnetic energy emitted is wide, ranging from infra-red to UV. Most of this energy is converted into light invisible to the human eye, resulting in an extremely inefficient light source. Halogen lamps can achieve up to 15 lumens per watt, although most are around 10 usable lumens per watt due to leakage and the trend that the lower efficacy filaments tend to have longer life span.
As stated, most Halogen globes today run on 12 VAC and therefore require some form of power transformer to reduce mains voltage. The earliest transformers were magnetic in nature, consisting of different ratios of coil windings around an iron core. In more recent times, electronic transformers have been developed. These transformers operate very differently (switch mode) to normal transformers, are generally more efficient, and have added safety features such as overload cut-out protection, and soft start-up.
It is an object of the invention to provide a power supply to provide a low power feed for articles such as low power lights.
It is also an object of the invention to provide a means and method for power supply which overcomes or at least ameliorates one or more problems of the prior art.